1. Field of the Invention
The present invention relates to an illumination optical apparatus, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
Recently, since a processing speed of a semiconductor is higher and electronic devices are scaled down, the requirement for miniaturizing semiconductor device patterns is higher and higher. In the process where fine circuit patterns are drawn on a substrate such as a silicon wafer or a glass plate, a photolithography technology is essential.
In the lithography process, a reticle (an original plate) on which a desired pattern is patterned in advance is illuminated and the image is transferred onto a photosensitive substrate via a projection optical system. Although the term of “a reticle” was used as an original plate in the above description, generally, “a reticle” is used when the projection optical system is a reduction optical system, and “a mask” is used when the projection optical system is a same magnification optical system. Although the present invention is not limited by a magnification ratio of the projection optical system, for easy understanding, hereinafter, the term of “a reticle” will be used.
The semiconductor exposure apparatus can be mainly categorized into two types of apparatuses that are a step-type exposure apparatus and a scan-type exposure apparatus. The step-type exposure apparatus has an advantage that the configuration is comparatively simple and the cost can be reduced compared to the scan-type exposure apparatus. However, a large exposure field of the projection optical system is needed for exposing a wider area. Therefore, it has a disadvantage in view of the aberration correction.
On the other hand, the scan-type exposure apparatus performs an exposure while synchronizing and scanning the reticle and the photosensitive substrate. It can expose an area lager than the exposure field of the projection optical system by scanning. Thus, it has an advantage that the size of the exposure field of the projection optical system can be reduced. Therefore, in view of the aberration correction of the projection optical system, the scan-type exposure apparatus is superior to the step-type exposure apparatus.
The resolution R of the exposure apparatus is generally represented by the following expression (1), which is called an expression of Rayleigh.
                    R        =                  k          ⁢                                          ⁢          1          ⁢                      λ                          N              ⁢                                                          ⁢              A                                                          (        1        )            
In the expression (1), k1 is a process coefficient, λ is a wavelength of a light source of an exposure apparatus, and NA is a numerical aperture of a projection optical system. In accordance with the expression of Rayleigh represented by the expression (1), the process coefficient k1 or the wavelength λ needs to be smaller, or the numerical aperture NA of the projection optical system needs to be greater in order to draw fine circuit patterns with small resolution R.
Recently, as a means for making the numerical aperture NA of the projection optical system greater, an immersion exposure technology or the like has an attracted attention. However, generally, if the numerical aperture NA of the projection optical system is greater, the projection optical system gets bigger or more complex, and the cost of the exposure apparatus increases. The cost of the exposure light source with a small wavelength λ is high. In using the exposure light source with a small wavelength λ, since the absorption rate and the birefringence of a glass material are large, there is a problem that the efficiency is decreased and a desired image quality can not be obtained.
As a means for making the resolution R small without changing the wavelength of the light source or the numerical aperture NA of the projection optical system, there is a method called RET (Resolution Enhancement Technology) which makes the process coefficient k1 small. For example, as one of the technologies, an auxiliary pattern or an offset of line width is provided on the reticle in accordance with the optical properties of the exposure optical system.
As a means for obtaining an effect which is equivalent to or more than such an optimization of the reticle, there is a means for optimizing, in accordance with the reticle pattern, the light intensity distribution on the pupil surface which has a relation of substantially a Fourier transform with the surface to be irradiated. This is generally called an off-axis illumination method. The light intensity distribution on the pupil surface which has a relation of substantially a Fourier transform with the surface to be irradiated and on the conjugate surface of the pupil surface is generally called an effective light source.
As an off-axis illumination, an annular illumination or a multipole illumination is the most common. The effective light source is often expressed by an illumination parameter of “outer σ”, “inner σ”, or “angular aperture” as shown in FIG. 8. The “outer σ” and the “inner σ” correspond to an outside diameter and an inside diameter of the effective light source, respectively. The “angular aperture” is an angle which is formed by an illumination part (an aperture part) when a point on the optical axis is an apex.
The parameter called an “annular ratio” is also often used. This is a ratio between the inner σ and the outer σ, and is defined as inner σ/outer σ. An optimal annular ratio is different in accordance with the reticle pattern. Therefore, it is preferable that an exposure apparatus is configured to be able to adjust the annular ratio.
As a method of adjusting the annular ratio, for example, Japanese Patent Laid-Open No. 5-21312 discloses a method of positioning a stop which has an annular shape adjacent to a pupil surface.
As another method of adjusting the annular ratio, Japanese Patent Laid-Open No. 5-251308 discloses a method of using a prism which has a refractive surface of a conical shape. It describes that it is preferable that the apparatus is configured to be able to set the annular ratio in a range of ⅓ to ⅔ by adjusting the space between a concave prism and a convex prism.
Furthermore, as another method of adjusting the annular ratio, Japanese Patent Laid-Open No. 2001-35777 discloses that a focal distance variable optical system, a conical prism, a space adjustable axicon, and a zoom lens are provided.
The method of positioning a stop which has an annular shape adjacent to a pupil surface as disclosed in Japanese Patent Laid-Open No. 5-21312 has a problem that the efficiency of the light which is used for exposure decreases in order to shield the light beam. Furthermore, there is another problem that a continuous adjustment of the annular ratio can not be performed.
According to the method of simply adjusting the space between the concave prism and the convex prism as disclosed in Japanese Patent Laid-Open No. 5-251308, for example, it was difficult to achieve the annular ratio variable in the range of ½ or lower to ¾ or higher. In this method, in order to widen the variable range of the annular ratio, it needs to make an inclined angle of the prism greater or to make a moving distance greater. However, when the inclined angle of the prism is large, the transmittance of the light gets small by the influence of reflecting or scattering of the light and enough efficient can not be obtained. Furthermore, when the moving distance of the prism is greater, the apparatus gets larger.
A method as disclosed in Japanese Patent Laid-Open No. 2001-35777 also needs a lot of optical elements to be positioned in an optical path. Therefore, the apparatus gets lager.
Thus, in the conventional technology, there was a problem that the decrease in efficiency was caused or the apparatus got larger if the annular ratio was continuously adjusted in a wide range.